The case for investment in eye health: systematic review and economic modelling analysis

Abstract Objective To assess how the returns on investment from correcting refractive errors and cataracts in low- and middle-income countries compare with the returns from other global development interventions. Methods We adopted two complementary approaches to estimate benefit-cost ratios from eye health investment. First, we systematically searched PubMed® and Web of Science™ on 14 August 2023 for studies conducted in low-and-middle-income countries, which have measured welfare impacts associated with correcting refractive errors and cataracts. Using benefit-cost analysis, we compared these impacts to costs. Second, we employed an economic modelling analysis to estimate benefit-cost ratios from eye health investments in India. We compared the returns from eye health to returns in other domains across global health and development. Findings We identified 21 studies from 10 countries. Thirteen outcomes highlighted impacts from refractive error correction for school students. From the systematic review, we used 17 out of 33 outcomes for benefit-cost analyses, with the median benefit-cost ratio being 36. The economic modelling approach for India generated benefit-cost ratios ranging from 28 for vision centres to 42 for school eye screening, with an aggregate ratio of 31. Comparing our findings to the typical investment in global development shows that eye health investment returns six times more benefits (median benefit-cost ratio: 36 vs 6). Conclusion Eye health investments provide economic benefits with varying degrees based on the intervention type and location. Our findings underline the importance of incorporating eye health initiatives into broader development strategies for substantial societal returns.


Introduction
Although vision impairment affects 2.2 billion people, 1 eye health services are underfunded and undervalued in global health policy.Notably, Transforming our world: the 2030 agenda for sustainable development lacks any reference to eye health. 2he World Health Organization (WHO) estimates a funding gap of 14.3 billion United States dollars (US$) to address preventable vision loss, and notes poor integration of eye services within existing health systems. 1 Estimates from 2014-2018 shows that eye health investment from bilateral, multilateral and private philanthropic institutions was US$ 102 million annually, less than 0.06% of development spending. 3This limited acknowledgement of eye health as an important area for investment may have contributed to the lack of progress against the World Health Assembly goal of reducing avoidable vision impairment by 25% between 2010 and 2019. 4ne potential reason is that eye health investment, especially in addressing uncorrected refractive error, falls short of being one of the best health investments when comparing incremental cost per disability-adjusted-life-year (DALY) averted. 3,5,6educing DALYs at lowest cost is a widely accepted criterion for prioritizing health investments.However, this approach overlooks the potential additional benefits of interventions, such as productivity gains and broader welfare improvements.
Eye health interventions, which may have low DALYs averted yet prevent significant productivity losses, are consequently at a disadvantage when using a cost-per-DALY-averted criterion.For instance, the estimated disability weight for blindness indicates an 18.7% annual health loss. 7However, productivity loss, even just considering employment, averages 30.2% for those with blindness and moderate or severe visual impairment. 3,8onsidering refractive error, near and mild vision impairment disability weights indicate a 1.1% or 0.3% annual health loss, respectively. 7Studies providing glasses in agriculture, 9 manufacturing, 10 and school settings [11][12][13][14] suggest annual productivity losses that are approximately five to 20 times greater than corresponding health losses.Therefore the value of eye health interventions might be better reflected in productivity and other welfare gains, rather than by averted DALYs alone. 15he purpose of this paper is to demonstrate how productivity and other welfare outcomes can be incorporated into economic evaluations of eye health investment, focusing on refractive error and cataracts, the two largest sources of visual impairment. 1 By conducting a systematic review, we identified studies from low-and middle-income countries which have measured productivity, income, learning and other welfare impacts associated with improving vision from refractive error and cataracts.Using the outcomes of the identified studies, we calculated the benefit-cost ratio to assess the welfare benefits Research of the intervention against its costs.One significant advantage of benefit-cost analysis is that it encompasses various benefits, not limited to health impacts. 16dditionally, we employed an economic modelling analysis to estimate benefitcost ratios from eye health investments in India.

Systematic review
The protocol for the systematic review was registered and published on Open Science Framework. 17We conducted a literature search in PubMed® and Web of Science™ on 14 August 2023 with no language restrictions.Box 1 shows the inclusion criteria.We excluded studies that focused solely on health-related outcomes like DALYs, quality-adjustedlife-years, mental health outcomes and health expenditure.
Tw o au t h o r s i n d e p e n d e nt l y screened titles and abstracts to identify articles for full-text assessment.They then assessed the selected full-text articles to confirm that studies met the inclusion criteria.They resolved disagreements through discussion.If impossible, a third author acted as arbitrator.We also consulted recent reviews at the intersection of eye health and economics or education, 3,15,[18][19][20] and reference lists of studies meeting inclusion criteria in our systematic review.
We extracted data about the study population, the intervention and study country, as well as information necessary to conduct the benefit-cost analyses.All outcomes used in the benefit-cost analyses were assessed using the Cochrane risk-of-bias assessment relevant to the study design.Publication bias was assessed using a funnel plot.More details are available in the online repository. 21

Estimating benefit-cost ratios
We adopted two complementary approaches to estimating benefit-cost ratios from eye health investment.The first approach used results of studies identified via the systematic review, where researchers had documented differences in the productivity, income or learning impacts between groups with and without visual impairment, including those with corrected vision.Using individual studies provides greater confidence in the magnitude of impacts and benefitcost ratios, particularly when estimated using experimental approaches.
For the second approach, we used an economic modelling analysis of benefits, along with detailed micro-costing data to estimate the benefit-cost ratios from implementation of four screening interventions at scale in India.The economic modelling approach facilitates the exploration of scaled interventions; however, the benefit-cost ratios reported might be less precise.
For both approaches, we conducted benefit-cost analysis from a societal perspective and adopted a common set of assumptions to facilitate comparability.Costs and benefits were discounted at a 3% rate, following published guidelines. 16e assumed that eyeglasses would last for three years for adult populations and one year for school children.We assumed that the benefits of cataract surgery would last for the rest of the beneficiary's life.We sourced mean life expectancy of the study populations from World Health Organization (WHO) life tables. 22here we adopted costs or benefits from the same country in a different year, we converted values to the same currency year using gross domestic product (GDP) deflators from the World Bank Development Indicators. 23 Generally, values were reported in US$.Before making inflation adjustments, we converted prices to local currency using the exchange rate at the time the costs or benefits were assessed.Where we adopted costs and benefits from different countries, we used a cost-transfer approach which assessed the magnitude of cost in purchasing-power-parity (PPP) terms from the initial country as a percentage of GDP PPP, and applied that ratio for the same costs in the target country.
We compared the benefit-cost ratio distribution to a recent two-year project aimed to identify best-buy interventions in global health and development, 24 and the Copenhagen Consensus Center library of 652 benefit-cost analyses of interventions. 25We also compared our estimated ratios to representative benefit-cost ratios for noncommunicable disease 26 and nutrition interventions (online repository). 21e chose noncommunicable disease interventions because many eye health conditions are categorized as noncommunicable diseases.For nutrition, we chose these interventions, because their benefits might not be fully represented by traditional health-related cost-effectiveness metrics.Like eye health, the positive impacts of nutrition interventions can manifest in areas like current productivity, 27 future productivity 28 and learning. 29

Individual studies
In each analysis, we assessed productivity, income and learning benefits, measures that are easily converted into monetary units for benefit-cost analysis.We also sourced costs from the programme, if reported.Where costs were unavailable, we identified costs by searching PubMed® and Web of Sci-ence™ for articles describing the same intervention conducted in the same country, or if that proved infeasible, we identified costs from other countries including a recent review on the costs of treatment. 15Where we used costs from another country, we prioritized countries from the same region before adopting costs from alternative regions.Box 2 provides an example of estimating the benefit-cost ratio of providing eyeglasses to tea-pickers in India.
In practice, our benefits were usually narrow, incorporating only one benefit that was reported in the study, while costs were more comprehensive, implying conservative benefit-cost ratios.We calculated ranges using reported confidence intervals (CIs) on benefit parameters.Further details on estima-

Box 1. Inclusion criteria for studies used to calculate benefit-cost ratios for interventions improving vision from refractive error and cataracts
The study compared two groups cross-sectionally with and without refractive error or cataracts, where an impact variable is defined as the dependent variable in a regression, and there is an attempt to control for endogeneity such as instrumental variable design or propensity score matching; or a study that provided eyeglasses or cataract surgery to the study population, estimating outcomes over time with or without comparison to groups which were not provided with eyeglasses or cataract surgery.
The study measured and reported quantitative impacts in the following non-health welfare outcomes: (i) employment, productivity, income, consumption, expenditure, gross margins, profit and revenue at the individual, household or enterprise level; or (ii) academic performance in school children.tions from each study are available in the online repository. 21s per the pre-registered protocol, 17 we refrained from including multiple benefit-cost ratios for the same study populations.We prioritized impacts measured using the most rigorous methods, those with longest follow-up time, or those closest to the intervention's general equilibrium effect.We conducted a subgroup analysis examining benefit-cost ratios from studies that did not present a serious risk of bias.

Economic model
We based the economic model on two recently published studies from India: an economic modelling study of welfare losses associated with blindness and moderate and severe visual impairment; 32 and a micro-costing analysis of interventions delivered at scale by six eye health providers. 31We chose this example because the costing data allowed for analysis of eye health interventions delivered at scale.The micro-costing analyses were performed on four case-finding strategies used by six Indian eye health providers, screening 2.3 million people in a year. 31Values were reported as annualized figures in 2020 US$, assuming a 3% discount rate.
We estimated benefits, costs and benefit-cost ratios using data and parameters from the studies.The model included more than 30 parameters, the most important are noted in Table 1, and the full list is available in the online repository. 21By simultaneously varying parameters across 10 000 Monte Carlo simulations, we calculated probabilistic estimates using both STATA (StataCorp LLC, College Station, United States of America) and Microsoft Excel (Microsoft Corp., Redmond, USA).These steps reflect probabilistic sensitivity analyses conducted in the original studies upon which we derived the benefit-cost analysis. 31,32The simulation allows us to report 95% CIs on benefit-cost ratios.
We estimated benefits for interventions targeting the general population (vision centres, eye camps and door-todoor screening) using a welfare model. 32he welfare includes benefits of increased

Box 2. Example of estimating benefit-cost ratios of an eye intervention from individual studies
In Assam, India, providing eyeglasses to correct presbyopia in tea-pickers led to substantial productivity improvements: 5.25 kg more tea picked per day than a control group without corrected presbyopia. 9Over the 11-week harvesting season, with five working days per week, the amount of extra tea picked is therefore 289 kg per worker.Tea prices in Assam were no less than US$ 2.3 per kg in the period of the study (June to October 2017). 30The provision of eyeglasses yielded a productivity increase with an estimated market value of US$ 651 per year, or US$ 1842 for three years at a 3% discount rate.Discount rate 3% 16 Uniform, 0%-8% 32 Avoided loss in employment (moderate or severe visual impairment or blind) 30.2% reduction in employment for those aged 15-64 3,8 Uniform, 19.5%-43.5% 32 Avoided mortality (moderate or severe visual impairment) 1.26: 10-year all-cause mortality risk ratio relative to no visual impairment 3,33 Gaussian, mean: 1.26; SD: 0.06 3,33 Avoided mortality (blind) 1.90: 10-year all-cause mortality risk ratio relative to no visual impairment 3,33 Gaussian mean: 1.90; SD: 0.26 3,33 Improved productivity in employment Avoided 20% productivity loss 9,[34][35][36] Uniform: 17%-23% 9 Avoided caregiver costs (moderate or severe visual impairment) 5% of productive time for one person 37 Uniform: 2.5%-10.0% Avoided caregiver costs (blind) 10% of productive time for one person 37 Assumed as twice the draw for moderate or severe visual impairment Productivity improvement in domestic work (moderate or severe visual impairment or blind) 20% loss of productivity in household, nonmarket activities with value of loss estimated at 50% of wages 38 Productivity loss equal to draw from productivity loss in employment; value of time: uniform: 25%-75% 38 Increase in test scores from the provision of eyeglasses 0.18 standard deviation improvement [11][12][13] Uniform: 0.11-0.23 [11][12][13] Compliance rate of eyeglasses use, adults 0. Eye health investment Brad Wong et al.
employment, lower mortality risk, productivity gains in employment, welfare gains for the non-employed and avoided caregiver costs.The benefits for school screening are construed more narrowly as learning gains, where a standard deviation increase in test scores is associated with a 20% increase in adult income. 43osts are estimated as the sum of patient and provider costs.Patient costs include the cost to access screening, indirect costs of receiving cataract surgery, and periodic eyeglasses repurchase costs as necessary.Provider costs include the costs of case finding and treatment.Further detail on the estimation is available in the online repository. 21
Thirteen outcomes report on impacts associated with refractive error correction for school students.The remaining outcomes address household welfare impacts of cataract surgery, or productivity impacts of workers after refractive error correction.Seven studies (10 outcomes) were randomized controlled trials (RCTs); 14 studies (23 outcomes) were non-randomized observational studies.In terms of geographic coverage, 28 study outcomes were from Asian populations and five from sub-Saharan Africa.
Of the outcomes, we categorized four as having a low risk of bias, 10 as having a moderate risk of bias or showing some concerns, and the remaining 19 as having a serious risk of bias.We did not identify evidence of publication bias for a sub-group of studies focusing on school screening (online repository). 21

Benefit-cost ratios
Of the 33 outcomes identified from the systematic review, 17 were used in the benefit-cost analyses.46]58 We only used three outcomes in the benefit-cost analysis to avoid over representation of the same study populations.Several papers reported results of RCTs offering school screening and free eyeglasses for primary grade students in the Chinese provinces Gansu and Shaanxi in 2012 and 2013. 12,14,47The information provided in these papers was unclear on whether these three study populations represented separate or overlapping groups.We therefore report only the benefit-cost ratio associated with the median impact of these three studies; however including all three papers separately does not change the conclusion of this paper.There were two analyses that assessed impacts of refractive error on learning of students in Gansu province in 2004. 11,48Out of these two studies, we adopted impacts from the more rigorous study design, an RCT. 11Lastly, we did not proceed to benefit-cost analysis for one outcome, 57 due to the large, reported impact on learning which was 9.5 times larger than the median impact in reported education RCTs. 59he modelling analysis generated four benefit-cost ratios, for a total of 21 benefit-cost ratios.The ratios from both estimation approaches are presented in Fig. 2. Central estimates of benefit-cost ratios range from 2 for cataract sur-  Median benefit-cost ratio among the 21 estimates is 36 (mean: 40).The median benefit-cost ratio of 10 estimates derived from outcomes assessed as low, moderate risk of bias, or some concerns is also 36.CIs for interventions are generally large, with some studies generating CIs that contain benefit-cost ratios less than one.For the economic modelling approach, two benefit-cost ratios are above the median (school screening had a benefit-cost ratio of 42; and eye camps had a benefit-cost ratio of 38); and two benefit-cost ratios are below the median (door-to-door screening had a benefit-cost ratio of 29; vision centres had a benefit-cost ratio of 28).Table 3 presents costs, benefits and results from the economic modelling approach of four interventions in India.Total costs for one year of screening activities plus follow-up costs for eyeglasses replacement over a 10-or 20-year period equal US$ 31.6 million.The total annualized cost is US$ 2.1 million.All strategies have similar costs per person treated, ranging from US$ 4.2 to US$ 6.7 depending on the strategy, with an overall cost per person treated of US$ 4.5.The annualized benefits are estimated at US$ 67.4 million, and US$ 143 per person treated per year for all interventions.The benefitcost ratios vary from 28 for vision centres to 42 for school eye screening.The pooled benefit-cost ratio is 31.

Comparison to other interventions
By comparing our median benefit-cost ratio for correcting refractive error and cataract to 652 global development investments in low-and middle-income countries, we found that eye health investment returns six times more benefits (benefit-cost ratios: 36 vs 6) than the typical investment.Furthermore, our estimated benefit-cost ratio of 36 aligns with the range of benefit-cost ratios for the 12 best-buy global development interventions identified by the Copenhagen Consensus Center. 24The ratios for these interventions range from 18 for nutrition counselling, to 125 for electronic procurement systems.
Compared to noncommunicable disease and nutrition interventions, our studied eye health interventions had a higher median benefit cost ratio (36) than noncommunicable disease interventions (9) and nutrition interventions (13).However, the benefit-cost ratios for single interventions showed considerable varia-tion.For noncommunicable diseases, the benefit-cost ratio ranges from less than 1 to 102, for nutrition the range is 2 to 81, and for our examined eye health interventions it is 2 to 104.

Discussion
Our findings highlight that investing in eye health can yield substantial returns given that vision is integral to various societal activities, such as work, education and daily life.Improving vision has plausibly large impacts on productivity, learning and household income, while the costs of correction are comparatively small, typically around US$ 10 per year of improved vision (for non-school screening interventions) or per year of benefit from improved vision (for school screening interventions). 21o contextualize our results, we compared our estimated benefit-cost ratio with those of other global health development interventions.This comparison revealed that the returns from eye health investments are on par with best-buy interventions.However, the considerable variation in benefit-cost ratios for single interventions suggests that making blanket investment in domains like noncommunicable diseases,

Intervention (country) Benefit-cost ratio (95% CI)
Glasses for myopia, farmers, Cambodia (Glick et  Our results are relevant for decision-makers who may be motivated by a broader goal of improving citizens' welfare, rather than averting DALYs at lowest cost.Planning, finance and other sectors, or donors which fund multiple cause areas, should consider the potential of eye health interventions.Similarly, businesses and ministries responsible for productive sectors such as agriculture, manufacturing and services, could see vision screening as a means to enhance productivity.Additionally, the education sector might leverage vision screening to boost learning.Lastly, while eye health interventions do not typically avert DALYs at lowest cost, health ministries might consider the results presented here to justify expanded investment under broader decision criteria, as was the case for the nutrition sector in the 2010s (online repository). 21Furthermore, investment in eye health can reduce inequities, because uncorrected visual impairment is a determinant of poverty, 3,34 and the benefits of eye health interventions would most likely accrue in lower socioeconomic groups.
While the results demonstrate the potential of eye health as a highly beneficial use of resources, the study has limitations.First, the evidence stems primarily from selected country and sector contexts, excluding areas like Latin America and north Africa, and some countries with high visual impair-ment prevalence like Indonesia.Second, many studies presented a serious risk of bias.However, the median benefit-cost ratio remained consistent at 36, even when only considering studies with a low risk of bias, a moderate risk of bias, or presenting some concerns of bias.Uncertainty surrounding the benefits is evident from the broad confidence intervals.Third, in the studies we identified, associated costs were infrequently reported.When mentioned, the focus was typically on the costs of eyeglasses or surgery, with less attention given to other substantial costs like case-finding and patient expenditures.Furthermore, some of the benefit-cost ratios in this paper are based on a recent publication discussing the costs of eye health programmes in India. 31Considering India's pioneering role in delivering low-cost, high-quality eye care, it is uncertain how these costs translate to different contexts.Further research is needed to understand the variation in eye-care costs across diverse settings, ideally by examining large-scale programme implementations.
In our economic model, the parameter of 30.2% for avoided loss of employment is from a review of 15 countries, with nearly all being high-income countries. 8However, the parameter is within range of a worldwide survey of 256 286 people, who noted employment losses of 21% and 36% for self-reported severe and extreme visual difficulty, respectively. 60ence, the limited evidence suggests 30% loss in employment as used here is reasonable.
We assumed avoided caregiver costs to be 5% and 10% of productive time, respectively, for those having moderate and severe visual impairment and blindness.Despite the limited evidence for these parameters, we note that in a large-scale study in India, the average hours spent on caregiving for visual impairment was estimated at 4.6 hours per day, around 80% of a 40-hour working week. 61Studies from high-income countries indicate similar or greater hours of care required for those having blindness. 62his study underlines the need for further research.While our findings indicate the high potential of eye health investments, additional evidence is needed on how visual impairment affects sectors such as agriculture, manufacturing, and domestic work across different low-and middle-income country contexts.More research should focus on understanding the broader impact of vision correction, Résultats Nous avons identifié 21 études portant sur 10 pays.Parmi les résultats obtenus, 13 mettaient en évidence l'impact qu'une correction des défauts de réfraction représentait pour les étudiants.À l'issue de notre revue systématique, nous avons utilisé 17 des 33 résultats pour nos analyses coûts/avantages, le rapport médian s'élevant à 36.L'approche par modélisation économique pour l'Inde a débouché sur des rapports coûts/avantages allant de 28 pour les centres de la vision à 42 pour le dépistage des troubles de la vue en milieu scolaire, avec une proportion globale de 31.En comparant nos résultats avec les dépenses classiques en matière de développement mondial, nous avons constaté qu'investir dans la santé oculaire permettait de multiplier les avantages par six (rapport coûts/avantages médian: 36 contre 6).Conclusion Les investissements dans la santé oculaire s'accompagnent d'avantages économiques à différents niveaux, en fonction du lieu et du type d'intervention.Nos résultats montrent à quel point l'intégration d'initiatives de santé oculaire au sein de stratégies de développement plus vastes est importante dans la réalisation de bénéfices considérables pour la société.
The study was conducted in a low-or middle-income country The study was published between 1 January 2001 and 31 December 2022 inclusive.

Table 2 . Empirical literature reporting productivity and welfare impacts of visual impairment identified in the systematic review Study Study design, site; year of implementation Intervention description Study population and sample size Outcome for use in benefit-cost analysis Risk of bias assessment a
(continues. ..)Bull World Health Organ 2023;101:786-799| doi: http://dx.doi.org/10.2471/BLT.23.289863

site; year of implementation Intervention description Study population and sample size Outcome for use in benefit-cost analysis Risk of bias assessment a
using the Cochrane risk of bias tool for RCTs, which has three categories: low risk of bias, some concerns, high risk of bias.Observational studies were assessed using the Cochrane risk of bias tool for observational studies which has four categories: low risk of bias, moderate risk of bias, serious risk of bias and critical risk of bias.
a RCTs were assessed b Productive activities are household work, paid work, or work for own use.c Several townships were not compliant and provided control students with eyeglasses.These townships were excluded from the statistical analysis in the study's preferred results.(. ..continued) (continues. ..)Bull World Health Organ 2023;101:786-799| doi: http://dx.doi.org/10.2471/BLT.23.289863Eye health investment Brad Wong et al. gery in Ethiopia to 104 for providing eyeglasses for farmers in Cambodia.
: Benefit-cost ratios derived from individual studies identified via systematic review are labelled with the relevant study.The other four benefit-cost ratios are from the economic modelling approach from scaling interventions in India.CIs in economic modelling analysis are based on Monte Carlo simulations, varying key parameters simultaneously.CIs from individual studies based on reported variation in impact parameters.Results assume a 3% discount rate.Bull World Health Organ 2023;101:786-799| doi: http://dx.doi.org/10.2471/BLT.23.289863 CI: confidence interval.Notes

Table 3 . Estimated costs, benefits and benefit-cost ratios of refractive error and cataract case-finding strategies, India, 2019-2020
Results and intermediate values of an economic modelling study to estimate benefit-cost ratios from four case-finding interventions in India, conducted by six eye health providers during 2019-2020.Parameters are drawn from a study 32 estimating welfare losses associated with moderate or severe visual impairment and blindness, and a microcosting analysis in India.Bull World Health Organ 2023;101:786-799| doi: http://dx.doi.org/10.2471/BLT.23.289863 US$: United States dollars.a Net present value at 3% discount rate.Notes: 31 Results based on a 3% discount rate.Values are in 2020 US$.Bull World Health Organ 2023;101:786-799| doi: http://dx.doi.org/10.2471/BLT.23.289863